1. Field of the Invention
This invention relates, generally, to wastewater treatment systems; and particularly, to treatment systems which denitrify the wastewater.
2. Prior Art
Requirements imposed by numerous governmental agencies have generated several advances in wastewater treatment. Wastewater treatment facilities for the treatment of municipal or industrial waste are also under increasing budgetary pressures to employ systems which have lower capital, operating, and maintenance costs. Therefore, it is desirable to have a wastewater treatment system which minimizes costs while meeting the ever more stringent discharge quality requirements.
The treated discharge from wastewater treatment facilities must meet requirements for BOD (biological oxygen demand material), and in some cases must also meet requirements for maximum amounts of compounds containing nitrogen and phosphorus. The treatment process in which nitrogen is removed from wastewater is known as denitrification. While aerated zones are well-suited for reducing BOD by the use of aerobic organisms, such zones are not well-suited for denitrification. Non-aerated zones, which have a low content of dissolved oxygen (DO), are better suited for denitrification. Such non-aerated, low DO zones are also known in the art as anoxic zones.
In denitrification, activated sludge, also known as mixed liquor, is flowed into an anoxic zone. The mixed liquor contains aerobic organisms which have been growing because they have had fuel (BOD) and oxygen (found as DO in the mixed liquor). The aerobic organisms, upon entering the anoxic zone, find fuel (BOD) but find little or no oxygen. The aerobic organisms look to the nitrogen compounds as a source of oxygen. When the aerobic organisms break down the nitrogen compounds to obtain oxygen, nitrogen gas and other harmless bi-products are produced.
Different techniques have been used to create anoxic zones. In some systems, an anoxic zone is created in the oxidation ditch, usually just before the mixed liquor is aerated. Such systems are described in U.S. Pat. No. 4,290,884 to Mandt, and U.S. Pat. No. 5,275,722 to Beard. In other systems, such as that described in U.S. Pat. No. 3,764,523 to Stankewich, Jr., aerated treatment vessels are alternated with non-aerated anoxic treatment vessels.
There are at least two difficulties encountered in anoxic zone denitrification. First, it can be difficult to maintain the proper ratio among the three ingredients necessary to sustain the process: aerobic microorganisms (from the mixed liquor), nitrogen compounds (used for the oxygen they contain), and BOD (the fuel for the aerobic microorganisms). Secondly, it is difficult to achieve plug flow. Perfect plug flow is achieved when every unit of wastewater is cycled through a vessel for the exact period of time.
Because of friction along the sides of a vessel and other factors, achieving perfect plug flow is impossible; but it is desirable to come as close as possible to achieving it. The farther a denitrification system is from achieving perfect plug flow, the greater the probability that some wastewater is leaving the anoxic zone without having been properly treated, and some wastewater is remaining in the anoxic zone beyond the length of time necessary for denitrification. Neither of these conditions is desired.